If I had to make a list based on my own preferences in music...
1. Outkast
2. Wu Tang
3. The Roots
4. A Tribe Called Quest
5. Gang Starr
Honorable mentions include Bone and Mobb Deep because when they are on, they are really on, but when they aren't consistent enough for my tasts. This list is stictly based on staying power, consistent quality, and my overall liking of their music. Its argueable that some liek ATCQ and Gang Starr along with Wu Tang have had much more of an influence on where good hip hop is today, but I didn't take influence into account.